I have a Raspberry Pi 3 Model B and I installed Raspbian on it using NOOBS. I want to use Python 3.6 only. Is it ok if I uninstall both Python 3.5 and 2.7? Will there be no system problems? If I yes, how do I uninstall them?
You have not told us which system you use so it is speculation as to whether you will have problems or not.
I believe Debian based systems use Python 2 for system purposes.
Use the SDcard copier to copy the SDcard, remove each python package one at a time. If anything goes wrong, you have your SD card. If nothing goes wrong, clear the other SD card.
I tried uninstalling them one by one but I encountered a problem. The system won't boot correctly. I think both versions of Python is needed for Raspbian to run correctly.